FIGS. 1A and 1B illustrate conventional electrically programmable fuse (“eFuse”) circuitry 10 during, respectively, programming of the eFuse 20 and measuring of the resistance of the eFuse to determine its programmed state. To facilitate programming eFuse 20, i.e., “blowing” the eFuse so as to change its electrical resistance, circuitry 10 includes a programming transistor 30 that switches on and off a relatively high-energy (voltage and current) programming signal from a suitable power source 40 in response to an appropriate gate control signal 50 (FIG. 1A). When programming transistor 30 is on, it allows the programming energy to flow through eFuse 20, thereby causing the resistance of the eFuse to change. During programming, power source 40 is usually at a voltage level of, e.g., 3.3V, with a current on the order of, e.g., 12 mA for a 130 nm technology.
Besides blowing eFuse 20, another use of circuitry 10 is to make analog measurements of the resistance of the eFuse before and after it is blown. Measuring the resistance of eFuse 20 before and after blowing are important characterization tools used to ensure that the desired programming of the eFuse is achieved. To make pre-blow measurements possible, the voltage of power source 40 has to be relatively low, such as 0.1V (FIG. 1B), so that the electrical energy available to eFuse 20 during measuring does not blow the eFuse.
In order to withstand the relatively high voltage of power supply 40, during programming, programming transistor 30 must be a long-channel, thick-oxide transistor (e.g., gate length=240 nm, gate oxide thickness=5.2 nm in 130 nm technology). Such large transistors are integrated into various technologies for use in high-voltage settings, such as 2.5V and 3.3V interfaces, and, therefore, are readily available. However, the threshold voltage of these large transistors is moderately high (e.g., 0.6V), so that when the logic “1” level of gate control signal 50 provided to programming transistor 30 is from a conventional, low-voltage supply (e.g., Vdd=1.2V), the “overdrive,” which is equal to voltage of gate control signal, Vg=1.2V, minus the threshold voltage of the transistor, Vt=0.6V, is not large. This requires programming transistor 30 to be physically very large to achieve the desired current.
Another unfavorable situation happens when the overdrive varies significantly as the threshold voltage varies with normal processing variation around a range between 0.5V and 0.7V. In the context of circuitry 10, this type of variation results in uncertainty in the programming current as the voltage threshold of programming transistor 30 varies. Therefore, programming can be unreliable. Too little current will not be able to program eFuse 20, and too much current will rupture the eFuse. Thus, neither case provides a desirable programming mode. It would be desirable to have the ability to “tune” the logic “1” level to ensure a suitable programming current.
A higher voltage, e.g., a voltage in a 2.0V-3.3V range, is required for the “1” level of the gate control signal 50 to enable the size of programming transistor 30 to be reduced, and also to reduce the uncertainty in the overdrive. In this case, programming transistor 30 would be sized for the actual value of the logic “1” level. For each of 2.5V and 3.3V, unique designs would be required when those voltage levels are available. However, not all integrated circuit chips require a separate high-voltage supply for any reason besides eFuse programming. Providing extra power supplies and extra package pins is expensive, at least from a design point of view and is, therefore, desirable to avoid.
Power source 40 takes on a high voltage during programming and, therefore, would be suitable as a source for gate control signal 50 of the programming transistor 30 during blowing of eFuse 20. However, power source 40 would not be suitable as a source for gate control signal 50 during the resistance measurement because the voltage of the power source is so low during the resistance measurement (e.g., 0.1V) that programming transistor 30 would not conduct at all. Accordingly, there is a need for a means to make high gate control signal 50 a voltage sufficient to blow eFuse 20, but to still enable the resistance measurement without providing an additional high-voltage supply.